YAG laser welding has features as mentioned below and is extensively used in the automatic welding process for automobiles.
1) A focused laser radiation is obtained and enables a welding operation with a low distortion and at a high rate;
2) Laser radiation absorption coefficient for metal materials is several times higher than for a CO2 laser, thus allowing an efficient welding operation. Since the wavelength of the laser radiation is on the order of 1/10 that of CO2 laser, attenuation caused by plasmas generated during the welding operation is reduced.
3) The laser radiation can be transmitted through a flexible optical fiber, and thus a handling is facilitated and the laser can be used with a multi-joint robot. The transmission to a location which is located as far as 100 meters is possible.
4) The laser radiation can be used in a time division manner or spatially split (or power divided) manner, and this allows the distribution of the laser radiation to a plurality of machining stations to achieve a high utilization efficiency.
On the other hand, the welding operation by using YAG laser alone involves the following issues:
1) YAG laser is used in the welding operation by condensing the beam radiation to achieve an increased power density. However, the condensed spot has a very small diameter on the order of φ0.3 to φ0.1 mm, and this causes the following inconveniences;
For butt welding, the presence of a gap between members to be welded which are disposed in abutment against each other (see FIG. 4a, for example) allows the laser radiation to pass through the gap, causing weld defects. This imposes a severe demand on the cutting of an end face of a member to be welded where a weld is to be formed as well as on the manner of constraining parts of the members located adjacent to the weld.
For lap welding, the width of faying interface of a bead which extends from a top member to a bottom member (see FIG. 4b, for example) is limited, preventing a satisfactory bonding strength from being obtained.
For fillet welding, a satisfactory depth of fusion (see FIG. 4c. for example) cannot be obtained in the similar manner as mentioned above, preventing a satisfactory bonding strength from being obtained.
Where a filler wire is concurrently supplied to a region subject to YAG laser welding for purpose of improving excess weld metal or surface bead, the weldability is degraded because the power is consumed in melting the filler wire.
2) YAG laser apparatus requires a high equipment cost or initial cost, and therefore, where a plurality of equipment must be provided as in an automobile production line, a prohibitive installation cost results.
Japanese Utility Model Laid-Open No. 82,194/1987 discloses a nozzle structure which is used where it is considered that the presence of plasmoid gasses and metal vapors or a plume within and above a keyhole produced in a member being welded as a result of the irradiation from a laser stands in the way to achieving a deep fusion as a result of absorption of the laser beam by the plume, the nozzle structure serving to blow a shield gas into the keyhole to drive the plume to an exhaust path formed outside a laser radiation path to be withdrawn and exhausted. This represents an approach to improve the efficiency of supplying the input laser heat to the member being welded.
Japanese Patent Publications No. 49,195/1981 and No. 51,271/1992 and Japanese Patent Applications Laid-Open No. 137,044/1978 and No. 287,060/2001 disclose a combined use of TIG and a laser where a TIG torch melts a member to be welded, and a laser beam is projected to a resulting molten pool to increase the depth of fusion. These represent an approach in which TIG forms a shallow, but an extensive molten pool while the laser achieves a narrow, but deep fusion. The TIG produces an extensive molten pool to increase the width of the molten surface of the member being welded 41, 42 to fill a butt welding gap, as illustrated in FIG. 4a, for example, while the laser produces a deep fusion to increase the depth of fusion between members being welded 45, 46, as illustrated in FIG. 4c. As a consequence, the welding efficiency is improved as compared with the use of the laser alone, while improving the welding quality.
However, it will be seen that only the fusion produced by the laser prevails at a deep position, limiting the width of the fusion and resulting in a reduced welded strength. As illustrated in FIG. 4b, for example, the width of faying interface between members being welded 43, 44 is limited in the butt welding operation, leaving much to be improved.